Large-scale circulation over and around the Northern Kerguelen Plateau

The Kerguelen Plateau is known to constitute a major barrier to the eastward-flowing Antarctic Circumpolar Current. However, there is limited knowledge on the regional circulation due to sparse observations, especially in the Northern Kerguelen Plateau between the Fawn Trough and the Kerguelen Islands. As part of the physical component of the multidisciplinary KEOPS cruise, systematic CTD measurements along three SW-NE oriented sections were made on the latter plateau cutting through the annual phytoplankton bloom across the eastern flank of the plateau into deep water. Major hydrographic features and geostrophic velocities perpendicular to the sections were first documented from these in situ observations. In order to interpret the results within the general circulation context and to trace the upstream pathways of the observed water masses, we established a synthetic picture of the large-scale circulation over and around the entire Northern Kerguelen Plateau, using the combination of new data from KEOPS with historical hydrographic data together with all available information on mid-depth and near-surface velocities derived from autonomous floats, surface drifting buoys, and satellite altimetry. This synthesis shows that the time-mean geostrophic flow over the shallow platform is sluggish (of the order of 3–5 cm s−1) with a general anticyclonic circulation roughly following the local bathymetry. This weak flow is bordered to the east by relatively strong (up to 18 cm s−1) northwestward flow along the eastern flank of the Northern Kerguelen Plateau due to a northward bifurcation of cold Antarctic waters of eastern Enderby Basin origin carried by the powerful Fawn Trough Current. We have shown the utmost importance of the general circulation for the spatial distribution of primary productivity in that the sluggish circulation over the complex shallow topography of the Northern Kerguelen Plateau may precondition the recurrence of annual blooms, in contrast to neighboring strong advective regimes without blooming in deep water.